One-way check valves of the so called "duck bill" type generally incorporate a hollow cylindrical body terminating in a duck bill which permits flow in one direction but checks flow in the opposite direction.
In some environments it is desirable to not only permit flow in one direction and check flow in the opposite direction, but to also provide for pressure relief when the flow pressure in the opposite direction exceeds a certain value.
U.S. Pat. No. 3,155,110 dated Nov. 3, 1964 discloses various embodiments of check-relief valves including a prior art mechanism that incorporates a coil spring and the improvement, which utilizes collapsible legs or feet. In both versions relief is provided by an axial shifting of the entire valve body.
U.S. Pat. No. 3,159,176 dated Dec. 1, 1964 discloses a check-relief duck bill valve in which pressure relief is accomplished, not by axial movement of the valve, but by the provision of a flexible umbrella portion which lifts off its seat at pressures in excess of some value to permit back flow around the outside of the body of the valve.
Obviously the use of a separate coil spring makes the valve assembly more complicated and expensive, while it is often impractical or even impossible to utilize a valve which relies upon axial movement of the entire valve or flow around the outside of the valve body to provide pressure relief.